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SANDWICH CONSTRUCTION:

Structural sandwich is a layered composite formed by bonding two thin facings to a thick core. It is a type of stressed-skin construction in which the facings resist nearly all of the applied edgewise (in-plane) loads and flatwise bending moments. The thin spaced facings provide nearly all of the bending rigidity to the construction. The core spaces the facings and transmits shear between them so that they are effective about a common neutral axis. The core also provides most of the shear rigidity of the sandwich construction. By proper choice of materials for facings and core, constructions with high ratios of stiffness to weight can be achieved.

A basic design concept is to space strong, thin facings far enough apart to achieve a high ratio of stiffness to weight; the lightweight core that does this also provides the required resistance to shear and is strong enough to stabilize the facings to their desired configuration through a bonding medium such as an adhesive layer, braze, or weld. The sandwich is analogous to an I-beam in which the flanges carry direct compression and tension loads, as do the sandwich facings, and the web carries shear loads, as does the sandwich core.

In order that sandwich cores be lightweight, they are usually made of low-density material, some type of cellular construction (honeycomb-like core formed of thin sheet material), or of corrugated sheet material. As a consequence of employing a lightweight core, design methods account for core shear deformation because of the low effective shear modulus of the core. The main difference in design procedures for sandwich structural elements as compared to design procedures for homogeneous material is the inclusion of the effects of core shear properties on deflection, buckling, and stress for the sandwich.

Because thin facings can be used to carry loads in a sandwich, prevention of local failure under edgewise direct or flatwise bending loads is necessary just as prevention of local crippling of stringers is necessary in the design of sheet-stringer construction. Modes of failure that may occur in sandwich under edge load are shown in figure 1-1.

Figure 1-1. Possible modes of failure of sandwich composite under edgewise loads: General buckling, shear crimping, dimpling of facings, and wrinkling of facings either away from or into the core.

Shear crimping failure (fig. 1-1B) appears to be a local mode of failure, but is actually a form of general overall buckling (fig. 1-1A) in which the wavelength of the buckles is very small because of low core shear modulus. The crimping of the sandwich occurs suddenly and usually causes the core to fail in shear at the crimp; it may also cause shear failure in the bond between the facing and core.

Crimping may also occur in cases where the overall buckle begins to appear and then the crimp occurs suddenly because of severe local shear stresses at the ends of the overall buckle. As soon as the crimp appears, the overall buckle may disappear. Therefore, although examination of the failed sandwich indicates crimping or shear instability, failure may have begun by overall buckling that finally caused crimping.

If the core is of cellular (honeycomb) or corrugated material, it is possible for the facings to buckle or dimple into the spaces between core walls or corrugations as shown in figure 1-1C. Dimpling may be severe enough so that permanent dimples remain after removal of load and the amplitude of the dimples may be large enough to cause the dimples to grow across the core cell walls and result in a wrinkling of the facings.

Wrinkling, as shown in figure 1-1D, may occur if a sandwich facing subjected to edgewise compression buckles as a plate on an elastic foundation. The facing may buckle inward or outward, depending on the flatwise compressive strength of the core relative to the flatwise tensile strength of the bond between the facing and core. If the bond between facing and core is strong, facings can wrinkle and cause tension failure in the core. Thus, the wrinkling load depends upon the elasticity and strength of the foundation system; namely, the core and the bond between facing and core. Since the facing is never perfectly flat, the wrinkling load will also depend upon the initial eccentricity of the facing or original waviness.

The local modes of failure may occur in sandwich panels under edgewise loads or normal loads. In addition to overall buckling and local modes of failure, sandwich is designed so that facings do not fail in tension, compression, shear, or combined stresses due to edgewise loads or normal loads, and cores and bonds do not fail in shear, flatwise tension, or flatwise compression due to normal loads.

The basic design principles can be summarized into four conditions as follows:

2. The core shall be thick enough and have sufficient shear rigidity and strength so that overall sandwich buckling, excessive deflection, and shear failure will not occur under design1loads.

3. The core shall have high enough moduli of elasticity, and the sandwich shall have great enough flatwise tensile and compressive strength so that wrinkling of either facing will not occur under design— loads.

4. If the core is cellular (honeycomb) or of corrugated material and dimpling of the facings is not permissible, the cell size or corrugation spacing shall be small enough so that dimpling of either facing into the core spaces will not occur under design1 loads.

The choice of materials, methods of sandwich assembly, and material properties used for design shall be compatible with the expected environment in which the sandwich is to be utilized. For example, facing to core bonding shall have sufficient flatwise tensile and shear strength to develop the required sandwich panel strength in the expected environment. Included as environment are effects of temperature, water or moisture, corrosive atmosphere and fluids, fatigue, creep, and any condition that may affect material properties.

Certain additional characteristics, such as thermal conductivity, resistance to surface abrasion, dimensional stability, permeability, and electrical properties of the sandwich materials should be considered in arriving at a thoroughly efficient design for the intended purpose.

Detailed procedures giving formulas and graphs for use in structural design are given in subsequent sections of this Handbook. The formulas and graphs can be used to determine dimensions of facings and core as well as necessary core properties for sandwich components under various types of loads. Graphs and formulas are presented in terms of general parameters, and are not for specific materials. Design procedures involving buckling are based on theoretical buckling coefficients. These coefficients are in fair agreement with average test results, but allowance can be made in the final design to account for the scatter characteristic of buckling test results, perhaps by choosing a slightly thicker core, so that buckling of the sandwich component does not occur at design load.

The facings of a sandwich part serve many purposes, depending upon the application, but in all cases they carry the major applied loads. The stiffness, stability, configuration, and, to a large extent, the strength of the part are determined by the characteristics of the facings as stabilized by the core. To perform these functions the facings must be adequately bonded to a core of acceptable quality. Facings sometimes have additional functions, such as providing a profile of proper aerodynamic smoothness, a rough non-skid surface, or a tough wear-resistant floor covering. To better fulfil these special functions, one facing of a sandwich is sometimes made thicker or of slightly different construction than the other.

Any thin, sheet material can serve as a sandwich facing. A few of the materials usually used are discussed briefly in the following:

2.1.1.1.1 Aluminum Alloys. –The stronger alloys of aluminum, such as 7075-T6, 2024-T3, or 2014-T6, are commonly used as facings for structural as well as for non-structural sandwich applications.

2.1.1.1.2 Steel Alloys. –Stainless steel sheets are finding increasing use as a facing material in airframe sandwich construction. The chief advantage of stainless steel sheet is its high strength at elevated temperatures. Alloys such as 18-8, 17-7PH, and PH15-7Mo are currently finding use because high stresses can be realized. The 18-8 alloys can be rolled to various degrees of hardness to produce high strength but it should be understood that a sheet rolled full hard has a longitudinal compressive yield stress about one-half of the compressive yield stress in the transverse direction. This discrepancy can be closed by subsequent stress relief. Alloys of the 17-7PH and PH15-7Mo are precipitation hardenable and can be strengthened by heat treatment–usually to condition TH1050.

2.1.1.1.3 Titanium Alloys. –Alloys of titanium are currently of interest as facing materials because of their high strength-weight ratios and because they can be utilized for moderately high temperature applications.

2.1.1.1.4 Magnesium Alloys. –Magnesium alloy sheets have been used only experimentally as facing materials, but may find increasing application because of their low density.

2.1.1.1.5 Nickel Base Alloys. –Nickel base alloys such as René 41 can be utilized for heat-resistant sandwich at temperatures of 1200°-1500° F. René 41 is a precipitation-hardening alloy that needs protection from the atmosphere during heat treating. The alloy can be welded.

2.1.1.1.6 Cobalt Base Alloys. –Alloys of cobalt with chromium, nickel molybdenum, and tungsten are available for use in moderately stressed applications at temperatures of 1000°-1800° F. Alloys such as L605 can be brazed, or fusion or resistance welded.

2.1.1.1.7 Columbium Alloys. –Columbium alloys D-36, D-43, and Cb-752 are suitable for use at temperatures up to 2500° F if they are protected from oxidation by thin suicide coatings. These alloys can be brazed in an inert atmosphere or can be welded; however, degradation can be minimized by joining parts by diffusion bonding.

2.1.1.1.8 Molybdenum Alloys. –Alloy TZM of molybdenum can resist temperatures up to 2800° F. Need for protection and means of joining parts are the same as for the columbium alloys.

2.1.1.1.9 Beryllium. –The low weight and high elastic modulus of beryllium make it most attractive for use in sandwich composites. The metal is heat resistant in the range 1000°-1200° F. Parts can be joined by brazing or welding. Precautions must be taken to prevent individuals from inhaling toxic beryllium particles during fabrication of parts.

2.1.1.2.1 Glass-Fabric Reinforced. –Resin-impregnated glass-fabric facings possess acceptable properties for structural sandwiches when properly fabricated. Because of its excellent dielectric characteristics when fabricated with the proper resin, this type of facing is used almost universally for radomes of sandwich construction. A variety of weaves are available commercially, which makes it practicable, by orienting the fiber directions in the facing, to achieve a wide range of directional strength properties.

In many airframe applications, facings are exposed to moisture, either in the form of high humidity or free water. Even though the amounts of moisture absorbed by glass-reinforced plastic are quite small (on the order of 0.5 to 1.5 percent), the strength properties are decreased, with the amount of decrease depending upon the type of finish applied to the glass fabric. Current specification requirements permit only small losses of strength, after exposure to moisture, that are consistent with results of tests on fabrics made with more recent and effective finishes (such as Volan A, A-1100, Garan RS-49, T-31, NOL-24, and A-172). The most suitable finish for a given application is selected by the glass fabricator. For chemical resin types, such as phenolic, epoxy, and triallyl cyanurate-polyester resins, optimum properties are obtained by use of specific finishes with each resin formulation. The acceptable finishes for each approved resin are given in the qualified products lists that accompany the military specification for each chemical type of resin.

2.1.1.2.2 Glass Mats Reinforced. –Glass fibers are also commercially available in the form of mats, but owing to the relative non uniformity in thickness and resin content and because of the low strength when compared to glass fabric, mats have found little use in aircraft sandwich construction.